The present invention pertains to an ion mobility spectrometer (IMS) with gas chromatography (GC) column (GC-IMS) and internal controlled gas circulation, which can be used in trace gas analysis.
A gas analyzer with internal gas circulation has been known from DE-OS 198 56 784. A circulation filter for water vapor and higher-molecular-weight constituents of the gas, a circulating pump, a metering means for the inlet for the gas to be analyzed as well as a gas-chromatographic separation column to a closed circulation system are additionally arranged in a gas circulation of a concentration-dependent gas detector. The air of the internal gas circulation is used as the carrier gas utilizing the separation column of a suitable low admission pressure to distinguish components with equal mobility but different retention time and to suppress cross sensitivities. The supply of an external carrier gas can be eliminated.
However, many measurement problems in industrial practice require defined analysis times in agreement with technological requirements such as the rhythm of the measurement, the accuracy of the measurement and the sensitivity of the measurement.
According to the invention an ion mobility spectrometer with GC column and internal controlled gas circulation is provided. A flow of gas to be analyzed from a sample gas outlet of the IMS cell is split via a splitter into two partial flows. One branch has a pump and a analytical circulation filter. The smaller partial flow is sent to a sample gas inlet of the IMS cell via a switchable sample loop device for passing on or sampling and subsequently via a GC column. The larger flow of the gas to be analyzed is sent back from a splitter to a branch with a further pump, a circulation filter to an additional gas inlet of the IMS cell. An additional gas outlet of the IMS cell provides the flow back to the further pump as well as a pressure sensor and a temperature sensor of the larger flow gas circulation. This circulating gas flow is split internally in the IMS cell in a splitter into a drift gas flow and the internal flow of the gas to be analyzed.
The circulating flow may be split via a splitter arranged outside the IMS cell into the drift gas flow, which is sent into the cell via the inlet, and the flow of gas to be analyzed, which is sent to the branch.
The Ion mobility spectrometer may be provided that a splitter is provided in the flow of gas to be analyzed. A partial flow may be sent as a make-up gas flow via another splitter to the carrier gas flow. This partial flow is used for diluting the sample.
The invention makes possible the independent control of defined analysis times in agreement with technological requirements such as the rhythm of the measurement, the accuracy of the measurement and the sensitivity of the measurement in the embodiment of an analysis system operating with a closed gas circulation. The gas flow to be analyzed, which leaves the IMS cell, is sent over an additional pump and an additional filter. The gas flow to be analyzed is split downstream of the additional pump and an additional filter into two partial flows.
The larger partial flow is returned in a closed circuit to the area upstream of the pump. The other partial flow is sent via the sample loop (in the solenoid valve (MV) block) to the GC column and then to the sample inlet of the IMS cell. It is ensured as a result that the admission pressure before the GC column can be set sensitively and varied by varying the output of the pump. Disturbing pump shocks are eliminated by the filter. The splitting of the gas flow is provided because the GC column is able to process, in principle, only very small gas flows and a sensitive control based on the output of the pump is possible at relatively large flows only.
At the same time, the additional possibility of controlling the admission pressure of the GC column ensures the absence of reaction of the gas flow to be analyzed on the closed drift gas system that is formed by the circulating pump, the circulating pump filter, the drift gas inlet of the IMS cell and the drift gas outlet of the IMS cell, and the independence of a variation of the gas flow to be analyzed. The parameters in this circulation can also be varied based on the output of the circulating pump independently from reactions on the circulation of the gas to be analyzed. Sensors for the pressure and temperature provide data used for the control of the circulation parameters and the compensation of the measured values of the IMS detection by calculation. These sensors may be additionally arranged in the drift gas circulation.
In particular, the following parameters and properties can be varied independently from one another due to this arrangement:
The present invention shall be described in greater detail below. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.